1. Technical Field
The present invention concerns a method and equipment for continuous or semi-continuous casting of metal, in particular directly-cooled (DC) casting of aluminium, comprising a mold with a mold cavity or chill that is provided with an inlet linked to a metal store and an outlet with devices for cooling the metal so that an object in the form of an extended string, rod or bar is cast through the outlet.
2. Description of the Related Art
Equipment of the above type is widely known and used for casting alloyed or unalloyed metal that is processed further down the production chain, for example for remelting or extrusion purposes.
A major challenge for this type of prior art casting equipment has been to achieve a segregation-free, smooth surface on the product cast. This has been particularly important for products in which the surface is not removed before processing.
Surface segregation is assumed to be caused by two principal phenomena:
                1. Inverse segregation: when the metal comes into contact with the chill, solidification will begin in a thin layer. This solidification will normally take place from the chill towards the center of the bar. When the metal makes the transition from the liquid to the solid phase, the volume will decrease at the outside and this must be replaced with alloyed melt from areas further inside the bar. This produces so-called inverse solidification because the segregation takes place towards the solidification front. This type of segregation typically produces a thin alloyed zone under the surface of the bar that is 10-20% higher in alloy elements than the nominal alloy content.        2. Blooms: when the solidified shell on the outside of the bar is not in physical contact with the chill wall, alloyed metal may be pressed out through the solidified or partially solidified shell (remelting). This solidification produces a thin, highly alloyed zone outside the original surface and a corresponding depleted zone under the original surface.Inverse segregation is assumed, in turn, to be affected by:            1. Heat transfer from the bar to the chill walls.    2. The length of the contact zone between the chill and bar.    3. Grain refinement and solidification morphology.    4. Flows near the surface of the bar and their effect on the thermal field.    5. The alloy's specific properties (for example, thermal conductivity and solidification path).Moreover, blooms are assumed to be affected by:    1. Heat transfer from the bar to the chill walls.    2. The distance between the contact zone in the chill and the water strike point.    3. Solidification morphology and grain refinement.    4. Stationary and periodic deformations of the outer shell (sponge effect).    5. Pressure differences over the solidified/semi-solidified shell.    6. Flows near the surface of the bar and their effect on the thermal field.    7. The alloy's specific properties (for example, thermal conductivity and solidification path).To reduce segregation, the following are assumed to be important:    1. Reduced heat transfer between the chill and the bar. This also includes reduced friction between the chill wall and the bar.    2. Optimal distance between the start of the contact zone and the water strike point (must be adjusted in relation to the casting parameters and heat transfer between the chill and the bar).    3. Reduced metallostatic pressure above or in the chill.    4. Reduced fluctuations in the metal level (produces less segregation and fewer variations in surface topography).    5. Avoidance of periodic fluctuations in the contact zone on account of varying gas pressure and volume in the gas pocket inside the mold. This produces the characteristic rings seen on the surface of metal bars or rods.
The only method in daily use that can result in a bar without surface segregation is electromagnetic casting, but this method requires high investment and extensive control systems. With electromagnetic casting, the pressure differences over the shell are cancelled, i.e. blooms disappear. At the same time, there is no contact between the metal and the mold wall and therefore no inverse segregation zone is formed either. Using conventional casting technology, it is possible to reduce both blooms and inverse segregation by reducing the effect of the chill's contact with the metal.
Using a so-called hot-top with supply devices for gas and oil in the solidification zone for the metal and where a gas cushion is formed under the hot-top, the contact zone with the chill and the heat transfer to the chill are reduced as the distance from the water strike point to the contact zone with the chill wall is reduced. A small inverse segregation zone will be achieved in this way. With this casting method, however, a relatively high metallostatic pressure is used so that there are still some blooms. In addition, the method produces pulsation on account of the gas supply, combined with periodic reduction from the chill wall, which produces an annular segregation process and also an annular topography on the rod.
Using a nozzle/pin or nozzle/float ball, the pressure difference over the solidified shell and the contact zone between the chill and the bar can also be reduced so that the surface segregation decreases. However, this is a method that is difficult to use optimally on account of individual regulation of molds and the safety aspect in that the metal flow may stop suddenly (clogged nozzles). With optimal casting conditions for surface segregation, water will then penetrate into the liquid aluminium and produce a risk of explosion. Therefore, most nozzle/pin processes are operated with a higher metal level in the mold than is optimal for reduced surface segregation, i.e. the motive force for segregation increases.